A small size planetary gear type transmission in which an input side and an output side have the same axis is frequently used in a vehicle. For example, a transfer apparatus for a full time four-wheel-drive vehicle disclosed in JP2002-321541A includes two sets of planetary gear mechanisms intended for reducing an overall length of the apparatus. The planetary gear mechanism normally includes three elements, i.e. a sun gear, a ring gear, and a planetary carrier which all have the same axis. Then, a transmission can be constituted by allocating functions of an input element, a reaction element, and an output element to the sun gear, the ring gear, and the planetary carrier, or by switching the functions therebetween. Further, multiple functions can be used or the functions can be combined with the other gear mechanism, thereby achieving a wide range of application.
In order to improve a vehicle performance, it is very important to reduce a total weight of a vehicle. In the planetary gear type transmission, a size of each component is restricted depending on an amount of rotation power to be transmitted. However, reductions in size and weight proceed by reconsideration of its design. Further reductions in size and weight are strongly desired while securing strength.
Thus, a need exists for a planetary gear type transmission with further reduced size and weight.